We expect hard-drive capacity to grow over time. Now that hard drives have jumped from 2TB to 3TB, however, upgraders face some challenges: These new hard drives may have issues in some drive enclosures and in older PCs, which aren't prepared to address the entire capacity of a 3TB model. Even if you can work such storage into your system, it may not be in a single 3TB volume, but in a 2.2TB volume and a 800GB volume instead.

The Problem: Legacy Addressing

The problem with deploying 3TB drives relates to older PCs (those more than a few months old, in most cases), and stems from the formula 2^32*512=2,199,023,255,552, or 2.2TB--a hard-drive addressing scheme found in legacy BIOSs and operating systems. In that formula, 2 indicates binary, 32 is the number of bits allowed in a legacy disk address, and 512 is the number of bytes in a legacy hard-drive data block. If the BIOS, drivers, I/O card, or operating system in your PC still plays by rules that involve this formula, you'll have issues installing and using a 3TB drive.

This situation could have been avoided if the entire computer industry had future-proofed after enduring the 137GB (28-bit) limit problems that cropped up around the turn of the millennium. In truth, most vendors did--with the notable exception of Microsoft. The company chose not to implement support for anything larger than 2.2TB drives in any of its 32-bit consumer operating systems--including Windows 7. Microsoft even omitted support from 64-bit XP. If you were looking for a reason to move to 64-bit Windows 7, here it is--courtesy of a not so subtle (or particularly gracious) invitation from the industry giant.

Fortunately, you can find drivers and utilities that allow you to use a 3TB drive as auxiliary storage with any flavor of Windows, XP or later. I say "auxiliary" because you can boot Windows from a 3TB drive only if it's 64-bit Vista or 64-bit Windows 7--and then, only if you have a PC with an EFI/UEFI BIOS. EFI is Intel's Extensible Firmware Interface, and UEFI (United EFI) is the nonproprietary version based on the 1.10 EFI spec. EFI assumes the hardware and operating system interfacing chores from the BIOS after startup. The technology has been around since--you guessed it--the turn of the millennium, when the 137GB problems surfaced. Unfortunately, with no mainstream OS support from Microsoft (which is actually on the UEFI board of directors), most motherboard vendors saw no reason to implement UEFI before now.

The EFI technology component that allows the full utilization of drives greater than 2.2TB is called the GPT, or the GUID (Globally Unique IDentifier) Partition Table. GPT replaces the older MBR (Master Boot Record) scheme (which is subject to the 2.2TB limit) and allows partitions of up to 9.2 zettabytes (2^70)--a large, barely comprehensible number. Windows XP and later versions of the OS support GPT, but, again, only 64-bit Vista and 7 support it during the boot process.

While Windows systems suffer from the MBR's constraints, Macs and 64-bit Linux machines have no issues using or booting from 3TB drives. Apple has supported EFI/GPT since it switched to Intel CPUs. Most 32-bit Linux distributions also support 3TB drives, without requiring an EFI/UEFI BIOS.

You can use the full capacity of a 3TB drive via USB--with a drive enclosure that supports it. The SATA-to-USB bridge chip inside the enclosure takes care of any addressing issues. This is why the first 3TB drive shipped last summer as an external model--a switch from the usual pattern of internal units shipping first.

Although you shouldn't have any issues attaching a prepackaged 3TB external drive, you may have problems if you buy a 3TB internal drive and try to add it to a USB-connected enclosure yourself. Read the fine print: Many of the bare enclosures say they support 3TB, but not all of them do.

Installing a 3TB Drive

If you have 64-bit Vista, 64-bit Windows 7, a Mac, 64-bit Linux, or in some cases 32-bit Linux, check to see if your system's BIOS supports 3TB. If doesn't, you'll need to obtain a BIOS update or to buy a newer motherboard that supports 3TB.

If you already have a 3TB drive, you can check to see whether that drive is being properly recognized by booting up the system and looking in the BIOS to verify that the capacity is correctly reported. For operating systems that don't support booting from 3TB, you'll need to boot from a smaller drive and use the 3TB drive as auxiliary storage. Perform the same BIOS check, and then download and employ any required drivers and/or partitioning software the drive maker supplied. If you use third-party partitioning software, make sure to use the GPT, not MBR, partitioning scheme.

Note that NTFS is limited to 2^32 clusters, also known as groups of sectors. That means you must format the drive with at least 1024-byte clusters, or you'll fall prey to the formula I talked about earlier. The default is 4096, which allows up to 16TB; however, converting FAT partitions to NTFS often results in 512-byte clusters. If you're trying to move a FAT partition to your new drive--don't. You're better off creating a new partition, reinstalling the OS, and restoring the files you backed up (it's always wise to back up before doing any partitioning operations). This is because the FAT partition will in all likelihood be misaligned with the Advanced Format of your new drive.

Tip: With drivers or programs that don't allow a full 3TB partition, you can use Windows dynamic volumes to combine two partitions into a single drive letter.

Advanced Format Alignment Issues

Though not strictly a 3TB issue, all of the shipping 3TB hard drives also use Advanced Format--a low-level storage scheme employed on newer drives. AF uses larger 4KB data sectors, an approach that improves performance and diminishes the number of addresses required for any given amount of data. If you transfer legacy partitions via image backups to an AF drive, or if you format an AF drive with XP, the older 512-byte sectors might not align correctly with the new scheme.

The performance hit caused by misaligned partitions is real: After we corrected a misaligned partition on one of the 3TB drives we tested, it improved its performance by nearly 30 percent on writing groups of small files and folders.

Utilities available from hard-drive vendors and from third parties such as Paragon Software will align existing partitions, but doing so can be a very slow process. Realigning my mostly full 1TB data drive with Paragon's Alignment Tool took upwards of 10 hours.

Hands On With 3TB Internal Hard Drives

The big three drive manufacturers--Hitachi, Seagate, and Western Digital--each sent us a 3TB hard drive for testing. All three drives were very good performers when writing and reading large files, as well as when reading small files and folders. Two of them, the Hitachi Deskstar 7K3000 and Seagate Barracuda XT, did well in writing large amounts of small files, too, while the Western Digital Caviar Green (the drive that had the aforementioned alignment issue) struggled slightly in this scenario. Overall, the Deskstar 7K3000 was the best performer, albeit by only about 10 percent over the Seagate Barracuda XT. The WD Caviar Green lagged a little behind, but keep in mind that the Green is oriented toward conserving power. It varies its rotational speed between writes, which hurts it when copying lots of smaller files--one part of our testing. (Click the chart below to view it at full size.)

Another differentiator between the drives turned out to be the software. Hitachi includes a driver/partitioning utility (from Paragon Software) that allows you to utilize all 3TB in a single partition. By contrast, as of this writing, the Acronis software favored by Seagate and WD allows you to use the entire drive capacity, but only in separate 2.2TB and 800GB partitions. To create a single volume, you need to follow the previously mentioned tip about combining partitions using Windows dynamic disks.

The Hitachi Deskstar 7K3000 was not only the best performer in PCWorld Labs testing, but also the best bargain. It sells for about $120, compared with $150 for the Western Digital Caviar Green and nearly $200 for the Seagate Barracuda XT. That makes the 7K3000 the only drive of the three that competes on a cost-per-gigabyte basis with 2TB hard drives. One fortunate byproduct of the introduction of 3TB drives is a commensurate drop in the price of 2TB drives: 2TB units are now selling for as low as $75.

With such prices, most users seeking to upgrade--especially those with legacy or 32-bit Windows operating systems--will probably be better off purchasing two 2TB drives. Although 3TB drives are great, the current hassles and the relatively high price per gigabyte limit their usefulness on systems lacking the appropriate technology.